Ohmaiga---- the sister was a spy----? No way-----
Yeah.
To be honest, if the protagonist doesn't see her as more than a sister, suddenly hugging him naked would creep him out so much...
Anyways, his options right now are to push her away or take the opportunity and do it
and then push her away (she's probably already gone through one of those *insert ero-doujin developments* anyways). I'm sure he isn't stupid enough to actually submit to her, but...